Leave secured by you, once up for union (6)
I believe the answer is:
equity
'for union' is the definition.
(I've seen this before)
'leave secured by you once up' is the wordplay.
'leave' becomes 'quit' (synonyms).
'secured by' is an insertion indicator (inserted letters are held or secured).
'you once' becomes 'ye' (historical version of 'you').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ye' written backwards gives 'ey'.
'quit' placed into 'ey' is 'EQUITY'.
(Other definitions for equity that I've seen before include "The ownership interest of shareholders in a company" , "Moral justice for the actor's union" , "Fair play - actors' union" , "Fairness; shareholding" , "Fairness or impartiality" .)